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I believe consensus is that the moon will eventually drift away from the Earth. Do such impacts, predominantly on the far side of the moon, apply enough force t

I believe consensus is that the moon will eventually drift away from the Earth. Do such impacts, predominantly on the far side of the moon, apply enough force to change that prediction? Are such projections of whether the moon leaves Earth orbit (or instead collides with Earth) accounting for this?

throwanem 4y ago
The moon is drifting away from Earth at a rate of about four centimeters per year [1]. Inbound impacts on the far side won't significantly affect this because of the way orbital mechanics works. To lower an orbital altitude, you need thrust opposing the orbital vector. (This is why spacecraft orient the way they do, with engines pointing in the direction of travel, for retrofire.) Thrusting toward or away from the orbital center of gravity (in this case Earth) can affect the circularity of the orbit, but that's all. In the case of the moon intercepting an impactor that'd otherwise strike Earth, the thrust vector is necessarily toward Earth, so the effect would be to render the moon's orbit infinitesimally more elliptical, with the increase along the major axis balanced by an equally minimal decrease along the minor axis. (To the extent the force vector of the impact is displaced from the axis through the moon's own center of gravity, that will affect the spin of the moon, but not its orbit.) According to the linked article, it will take about fifteen billion years for the moon to escape Earth orbit entirely. I haven't checked, but I believe that's considerably longer than it is expected to take for the Sun to expand as a red giant and render Earth uninhabitable. Certainly more wondrous than either event would be for anything then living to remember having once had distant ancestors who called themselves "human"... [1] https://www.google.com/amp/s/public.nrao.edu/ask/what-happens-as-the-moon-moves-away-from-the-earth/amp/ https://www.google.com/amp/s/public.nrao.edu/ask/what-happen...
- bdefore 4y ago
  It's compelling to me that the Earth and its moon are so tightly linked that their relationship is projected to outlast a life-providing sun. I believe this is moreso than any other moon-planet pairing in the solar system? Given what we know of incoming asteroids and comets, the probability of their impacts hitting with enough force at a trajectory you describe with enough significance to change the moon's orbit is expected to be low . And we're relatively confident of that?
  - throwanem 4y ago
    I don't have numbers or references here, but my intuition from a lifetime of space nerding is that, if the moon were struck with enough retrograde force to materially lower its orbit, in the best case that impact would liberate debris of mass and vector such that our first concern would be planetwide disaster response. (In the worst case, it might shatter the moon entirely and distribute a significant fraction of its total mass across Earth's surface; in such a scenario, the survival of eukaryotic life anywhere on Earth qualifies as a million-to-one-shot win.) But the odds of such a collision I suspect are significantly lower even than of a similarly sized impactor striking Earth. Granted that the moon helps a great deal in sweeping space near Earth of objects that might otherwise enter our atmosphere, but something with the kind of kinetic energy it would take to produce these outcomes isn't going to spend long enough within the gravitational influence of the Earth-Moon system to be significantly affected before impact. It'd be like trying to swerve an ICBM warhead by blowing really hard on it, and I choose that simile with care - if this ever does happen, I think it might constitute a clear indication that someone somewhere in the universe really hates us!

throwanem 4y ago

The moon is drifting away from Earth at a rate of about four centimeters per year [1]. Inbound impacts on the far side won't significantly affect this because of the way orbital mechanics works. To lower an orbital altitude, you need thrust opposing the orbital vector. (This is why spacecraft orient the way they do, with engines pointing in the direction of travel, for retrofire.) Thrusting toward or away from the orbital center of gravity (in this case Earth) can affect the circularity of the orbit, but that's all. In the case of the moon intercepting an impactor that'd otherwise strike Earth, the thrust vector is necessarily toward Earth, so the effect would be to render the moon's orbit infinitesimally more elliptical, with the increase along the major axis balanced by an equally minimal decrease along the minor axis. (To the extent the force vector of the impact is displaced from the axis through the moon's own center of gravity, that will affect the spin of the moon, but not its orbit.) According to the linked article, it will take about fifteen billion years for the moon to escape Earth orbit entirely. I haven't checked, but I believe that's considerably longer than it is expected to take for the Sun to expand as a red giant and render Earth uninhabitable. Certainly more wondrous than either event would be for anything then living to remember having once had distant ancestors who called themselves "human"... [1] https://www.google.com/amp/s/public.nrao.edu/ask/what-happens-as-the-moon-moves-away-from-the-earth/amp/ https://www.google.com/amp/s/public.nrao.edu/ask/what-happen...

bdefore 4y ago
It's compelling to me that the Earth and its moon are so tightly linked that their relationship is projected to outlast a life-providing sun. I believe this is moreso than any other moon-planet pairing in the solar system? Given what we know of incoming asteroids and comets, the probability of their impacts hitting with enough force at a trajectory you describe with enough significance to change the moon's orbit is expected to be low . And we're relatively confident of that?
- throwanem 4y ago
  I don't have numbers or references here, but my intuition from a lifetime of space nerding is that, if the moon were struck with enough retrograde force to materially lower its orbit, in the best case that impact would liberate debris of mass and vector such that our first concern would be planetwide disaster response. (In the worst case, it might shatter the moon entirely and distribute a significant fraction of its total mass across Earth's surface; in such a scenario, the survival of eukaryotic life anywhere on Earth qualifies as a million-to-one-shot win.) But the odds of such a collision I suspect are significantly lower even than of a similarly sized impactor striking Earth. Granted that the moon helps a great deal in sweeping space near Earth of objects that might otherwise enter our atmosphere, but something with the kind of kinetic energy it would take to produce these outcomes isn't going to spend long enough within the gravitational influence of the Earth-Moon system to be significantly affected before impact. It'd be like trying to swerve an ICBM warhead by blowing really hard on it, and I choose that simile with care - if this ever does happen, I think it might constitute a clear indication that someone somewhere in the universe really hates us!